A polyethylene naphthalate fiber including an ethylene-2,6-naphthalate unit as the main constituent shows a high strength, high elastic modulus and excellent thermal dimensional stability, and is a highly useful fiber as an industrial material. Particularly, in a field of composites that are reinforced by a polyethylene naphthalate fiber, in particular rubber reinforcing materials and the like including a tire cord, it is expected as a material that exhibits performance exceeding a polyethylene telephthalate fiber that is generally used currently.
However, the molecule of a polyethylene naphthalate fiber is stiff and tends to be oriented in a fiber axis direction. Therefore, it has such drawback that, in the case where only high magnification stretch and heat treatment are performed, fatigue properties for repeating stress is lower as compared with other general-purpose synthetic fibers and mechanical properties under real use conditions is lowered.
In order to solve such problems, for example, Patent Document 1 discloses a polyethylene naphthalate fiber having a large Silk factor, which is obtained as (strength)×(square root of elongation degree), and a method for producing the same by defining stretch conditions of a first stage and a second stage. Patent Document 2 discloses a method for producing polyethylene naphthalate with an excellent toughness by defining conditions of a spinning cylinder just after spinning to delayed-cool the discharged yarn. However, there is a limitation on increasing the toughness of raw yarns, and it is important to improve fatigue properties of fibers in order to enhance the mechanical performance in a composite at real use.
For fatigue resistant properties, Patent Documents 3 and disclose polyethylene naphthalate fiber formed by copolymerizing a cyclic acetal or a bis-(trimellitimide) compound. However, although fatigue properties are improved by copolymerizing such a bulky third component, there is such a drawback that the strength thereof lowers because fiber structure is disturbed. Therefore the fiber could not be substantially applied to a fiber for rubber reinforcement such as tire cords.    Patent Document 1: JP-A-4-194021    Patent Document 2: JP-A-6-128810    Patent Document 3: JP-A-2003-193330    Patent Document 4: JP-A-11-228695